1. Field of the Invention
The present invention relates to a semiconductor wafer exposure apparatus and to a method of using the same to expose a photosensitive film formed on the wafer.
2. Description of the Related Art
During the fabricating of semiconductor devices, wafers are subjected to a sequence of different manufacturing processes. In the course of such a sequence, the wafers go through changes in temperature from room temperature to a temperature of more than several hundred degrees centigrade. Accordingly, the wafers experience thermal expansion and contraction during the processes.
Ideally, the physical characteristics of wafers should be maintained throughout the sequence of manufacturing processes. However, the physical characteristics of wafers change due to thermal expansion and contraction thereof during the different manufacturing processes. This causes an error in aligning the wafers with optical equipment during photolithography. Such an alignment error, if in excess of a predetermined limit, results in a defective final product.
More specifically, in the fabricating of semiconductor devices, a photolithographic process includes coating a wafer with photoresist, and aligning, illuminating and developing the wafer. FIG. 1 shows a conventional aligning exposure apparatus
Referring to FIG. 1, reference numeral 10 designates a spin coater for coating a wafer with a photoresist. The wafer coated with the photoresist by the spin coater 10 is transferred to an exposure apparatus 12. The exposure apparatus 12 comprises a wafer transfer system 14, a wafer pre-alignment system 16, a wafer stage 18 and a D-chuck 20. The wafer coated with the photoresist film by the spinner 10 is transferred via the wafer transfer system 14 to the wafer pre-alignment system 16. The wafer is pre-aligned by the wafer pre-alignment system 16 before being transferred to the wafer stage 18. Specifically, a flat zone of the wafer is oriented in a predetermined direction. The pre-aligned wafer is transferred to the wafer stage 18 and is then accurately aligned with optics of the photolithography exposure equipment. Thereafter, an exposure process is executed. In the exposure process, the same pattern as that carved in a reticle is transferred to the film of photoresist on the wafer. Then the wafer is transferred via the D-chuck 20 and the wafer transfer system 14 to the spin coater.
FIG. 2 is a sectional view of part of the wafer pre-alignment system 16 of the conventional wafer aligning exposure apparatus 12. Referring to FIG. 2, the wafer pre-alignment system 16 comprises a P-chuck 22 on which a wafer is loaded, and mark and edge sensors 23 and 24 disposed around the P-chuck 22. The edge sensor.24 senses is a flat zone of the wafer W loaded on the P-chuck 22. The mark sensor 23 is used to orient the sensed flat zone in the predetermined direction. The edge sensor 24 generates heat while sensing the flat zone of the wafer W, and therefore, the temperature of a portion of the wafer W exposed to the edge sensor 24 increases.
As described above, elements of the conventional aligning exposure apparatus have heat generation elements which increase the temperature of a wafer. In the wafer aligning exposure apparatus 12, the wafer transfer system 14, the wafer pre-alignment system 16 and the wafer stage 18 have different temperature environments. For example, the wafer stage 18 consistently maintains its temperature using a special temperature control device. On the other hand, the wafer transfer system 14 and the wafer pre-alignment system 16 use the air supplied from a cleanroom to control their temperatures. Not only does air heated by various heat generation sources in the cleanroom flow into the systems, but also each system itself of the wafer aligning exposure apparatus 12 has a heat generation element such as the edge sensor 24. Thus, the inside of each system of the aligning exposure apparatus is heated. Furthermore, since different aligning exposure apparatuses are used for different processes during the fabrication of semiconductor devices, different wafer transfer systems and different wafer pre-alignment systems are used.
For these reasons, the wafer which passes through the wafer transfer system 14 or the wafer pre-alignment system 16 in the exposure apparatus 12 is heated, and therefore, the temperature of the wafer is higher than a temperature maintained at the wafer stage 18 when the wafer arrives at the wafer stage 18. This means that the wafer has thermally expanded to a much greater extent than it would have thermally expanded at the wafer stage 18. Accordingly, the alignment error of the wafer becomes so great that the pattern of the reticle cannot be accurately transferred to the wafer. To prevent this problem, before the alignment and exposure processes are carried out, the wafer is caused to remain at the wafer stage 18 for a long time sufficient for the wafer and the wafer stage 18 to attain a state of thermal equilibrium. However, requiring the wafer to remain idle for a long period of time compromises the productivity of the semiconductor fabricating process.
It is therefore an object of the present invention to prevent excessive thermal expansion of a wafer caused by a difference in temperature between a wafer stage and the wafer, without requiring the wafer to remain idle at the wafer stage for a prolonged period of time, thereby reducing the possibility of an alignment error without compromising the productivity of the manufacturing process.
To achieve this object, the present invention forcibly cools the wafer before it is transferred to the wafer stage.
To this end, the exposure apparatus of the present invention includes a wafer transfer system, a wafer pre-alignment system comprising a P-chuck, mark and edge sensors disposed around the P-chuck, and a wafer cooling unit for cooling a wafer while it is being pre-aligned, a wafer alignment system including a wafer stage, and a D-chuck.
The wafer cooling unit is installed over the P-chuck between the mark sensor and the edge sensor.
The wafer cooling unit is in the form of an air showerhead which sprays a cooling gas onto the entire surface of a wafer loaded on the P-chuck.
In a method of the present invention, a wafer is transferred from a spin coater to a wafer transfer system and from there to the wafer pre-alignment system.
The cooling of the wafer is preferably performed at the pre-alignment system while the wafer is being pre-aligned and until the temperature of the wafer becomes equal to a predetermined temperature prevailing inside the wafer alignment system.
The wafer is then transferred to the wafer stage of the alignement system where it is aligned with the optics of the photolithography exposure equipment and is then irradiated with light produced by a light source of the exposure equipment, whereby a photosensitive film formed on the wafer in the spin-coater is exposed.
As described above, the present invention forcibly cools the wafer in the wafer pre-alignment system to make the temperature of the wafer equal to that of the wafer stage just before it is transferred to the wafer stage. Consequently, the thermal expansion of the wafer coincides with that which would occur at the wafer stage, thereby preventing wafer alignment errors from occurring.